Continuous time analogue/digital converter

ABSTRACT

Continuous time analog/digital converter, comprising a sigma delta modulator (MSD 1 ) configured to receive an analog input signal (x(t)) and comprising high-pass filtering means (MF) the chopping frequency of which is equal to half of the sampling frequency (Fs) of the quantization means (QTZ) of the modulator (MSD 1 ).

This application is a continuation of U.S. patent application Ser. No.13/024,729, filed on Feb. 10, 2011, and entitled “Continuous TimeAnalogue/Digital Converter,” which application further claims thebenefit of French Patent Application No. FR 10-51011, filed on Feb. 15,2010, and entitled “Continuous Time Analogue/Digital Converter,” whichapplications are hereby incorporated herein by reference to the maximumextent allowable by law.

TECHNICAL FIELD

The invention relates to the continuous-time conversion of analoguesignals to digital signals and applies advantageously but not limitinglyto the radio-frequency field, for example in mobile telephony, in whichthe radiofrequency circuits mostly use analogue/digital conversiondevices both in transmission and reception.

BACKGROUND

These conversion devices are usually produced within integratedcircuits. They may be continuous time (or CT according to the termcommonly used by those skilled in the art) or else discrete time (or DTaccording to the term commonly used by those skilled in the art).

Usually, this conversion is carried out with the aid of a sigma deltamodulator because it provides a good resolution of conversion andrejects quantization noise outside the payload band of the signal.

FIG. 1 illustrates schematically an example of a structure usually usedfor sigma delta modulators within a continuous time converter CT. Thestructure of sigma delta modulators is usually based on the combinationof an integrator and a summer, the assembly being looped. The diagram ofa sigma delta modulator is therefore that of a closed-loop controlsystem.

The sigma delta modulator MSD comprises at the head a summer SOM(subtractor) receiving an analogue input signal x(t) having a frequencyFe. This subtractor is in this instance followed by an integrator INTthe output of which is connected to the input of a quantization meansQTZ (sampler) the output of which forms the output of the modulator. Theoutput of the quantization means QTZ is looped back to the negativeinput of the summer SOM by means of a digital/analogue converter DAC.The quantization means QTZ converts the signal from the integrator INTinto a digital signal y(n). Moreover, the modulator MSD comprises aclock generator CLK in order to generate a sampling frequency Fsintended for the quantization means QTZ and for the converter DAC.

The “delta” modulation is based on the quantization of the modificationof the analogue input signal x(t). The presence of a “sigma” integratorin the modulator gives the modulator the title of “sigma delta”modulator. The integrator usually comprises a low-pass filter

This being so, there are also bandpass sigma delta modulators whichcomprise, instead of an integrator, a bandpass filter. These modulators,although they have no integrator, still retain, the title “sigma deltamodulator”.

In practice, in radio-frequency applications, use is made of a bandpassfilter and a sampling frequency Fs which is approximately equal to fourtimes the frequency of the analogue input signal x(t). Since thesampling frequency is very high, this modulator MSD does not make itpossible to convert high-frequency analogue signals; usually thefrequency of the input signals is limited to a few gigahertz.

Moreover, as indicated above, in order to convert the analogue signals,it is also possible to use a discrete time analogue/digital converterDT. These converters also use sigma delta modulators but unlike thecontinuous-time use described above, these modulators receive a sampledsignal as an input. These converters also comprise a first frequencymixer situated upstream of the modulator in order on the one hand tosample the analogue input signal and on the other hand to transpose thefrequency of the input signal around a carrier frequency. Then themodulator converts the sampled signal received into a digital outputsignal. This converter also comprises a second mixer situated at theoutput of the modulator in order to again transpose the frequency of thedigital output signal around the frequency of the input signal.

Sampling the analogue input signal makes it possible to use a samplingfrequency value for the sigma delta modulator that is lower than thatused in the continuous time converter CT described above. Specifically,for a discrete time converter DT, the sampling frequency isapproximately equal to twice the frequency of the analogue input signal.

These discrete time converters DT do not make it possible to convert theanalogue signals at frequencies higher than the continuous timeconverters CT. Moreover, they have a complex architecture because theyuse two frequency mixers.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides for a continuous timeanalogue/digital converter. The converter includes a sigma deltamodulator configured to receive an analogue input signal. The modulatorincludes a quantization means having a sampling frequency; and ahigh-pass filtering means having a chopping frequency that is equal tohalf of the sampling frequency of the quantization means.

In another aspect, the present invention provides for a wirelesscommunication device having a sigma delta modulator configured toreceive an analogue input signal. The modulator includes a quantizationmeans having a sampling frequency, and a high-pass filter having achopping frequency that is equal to half of the sampling frequency ofthe quantization means.

In yet another aspect, the present invention provides for a methodcomprising: receiving an analogue input signal and adding to theanalogue input signal a feedback signal to generate a modified analoguesignal. The method further includes passing the modified analogue signalthrough a high-pass filter and filtering out a portion of the analoguesignal having a frequency below a chopping frequency, the choppingfrequency being approximately half a sampling frequency, to produce afiltered analogue signal. The method further includes quantizing thefiltered analogue signal at the sampling frequency to produce a digitalsignal, and converting the digital signal to an analogue signal togenerate the feedback signal.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a delta modulator used in the continuous timeconverters CT according to the prior art;

FIG. 2 illustrates schematically an embodiment of a continuous timeanalogue/digital converter according to an embodiment of the invention;

FIGS. 3 and 4 illustrate schematically other embodiments of a continuoustime analogue/digital converter; and

FIG. 5 illustrates schematically an example of a wireless communicationdevice incorporating a continuous time converter.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Before addressing specific embodiments in detail, a brief overview ofthe various embodiments, and potential advantages thereof is provided.According to one embodiment, a system is proposed for convertinganalogue signals at the highest possible frequency that is not verycostly and less complex than those described above.

Advantageously, a converter is proposed that can directly convert ananalogue signal with the aid of a sigma delta modulator that is adaptedwithout having to process the analogue input signal upstream of themodulator. Moreover, it may be advantageous to reduce the low-frequencynoise associated with the processing of the analogue signal during itsconversion to a digital signal. According to one aspect, a continuoustime analogue/digital converter is proposed that comprises a sigma deltamodulator configured to receive an analogue input signal. This modulatorcomprises high-pass filtering means the chopping frequency of which isequal to half of the sampling frequency of the quantization means of thesaid modulator. Therefore, the analogue input signal is converteddirectly at high frequencies, reducing the complexity of the converter.

Advantageously, no mixers are used for processing the input signalupstream of the modulator. By virtue of a high-pass filtering means, thebandwidth of the converter is increased for a given sampling frequencyof the modulator. The sampling frequency of the sigma delta modulatormay advantageously be approximately equal to twice the frequency of theanalogue input signal. The high-pass filtering means may be producedwith the aid of a simple high-pass filter or else from one or moreresonators.

Therefore, according to one embodiment, the modulator comprises at leastone summer, and the filtering means comprise at least one unitcomprising a resonator and a variable-gain amplifier coupled between theoutput of the quantization means and an input of the at least onesummer, the gain of the said variable-gain amplifier being fixed at avalue for which the resonance frequency of the said resonator is equalto the said chopping frequency.

Therefore, by the use of a resonator, the accuracy of the filteringmeans is increased, that is to say that the low-frequency noise is moreeffectively pushed outside the payload frequency band of the analoguesignal. It will be noted that this accuracy increases with the number ofresonators used. The resonators make it possible to obtain a signal gainat the resonance frequency of the resonator which is higher than thegain of a conventional high-pass filter. Moreover, variable-gainamplifiers make it possible to adapt the sigma delta modulator accordingto the frequency of the input signal. According to another embodiment,the unit comprises at least one other variable-gain amplifier, the gainsof all the amplifiers of the said unit being respectively fixed atvalues for which the resonance frequency of the said resonator is equalto the said chopping frequency. This improves the accuracy of eachresonator. The modulator may also comprise a delay means coupled betweenthe output of the quantization means and the input of one of theamplifiers of the said unit and configured to delay the digital outputsignal of the quantization means by a delay equal to half of thesampling period of the quantization means.

According to yet another embodiment, the filtering means comprise atleast one other unit. The converter as defined above may be produced inintegrated form within an integrated circuit. The various components ofthe converter may be produced in the form of specific logic or othercircuits, or else in the form of software modules within amicroprocessor. It will be noted here that the design of the high-passfilters on the integrated circuits, in particular in CMOS technology, iseasier to achieve than for bandpass filters.

According to another aspect, a wireless communication device is proposedcomprising a converter as defined above.

FIG. 2 shows schematically a continuous time analogue/digital converteraccording to one embodiment of the invention comprising a sigma deltamodulator MSD1 in which the sampling frequency Fs of the quantizationmeans QTZ is approximately equal to twice the frequency Fe of theanalogue input signal x(t). Moreover, the sigma delta convertercomprises a decimation filter, not shown for simplification purposes,coupled to the output of the quantization means QTZ. The modulator MSD1comprises high-pass filtering means MF. These high-pass filtering meansMF comprise in this instance a high-pass filter FPH the choppingfrequency Fc of which is approximately equal to half the samplingfrequency Fs of the quantization means QTZ. This high-pass filteringmeans also makes it possible to eliminate the low-frequency noise.

FIG. 3 illustrates schematically another embodiment of the continuoustime analogue/digital converter. In this embodiment, the filtering meansMF comprises a unit comprising a resonator RES, a first amplifier A1with variable gain a1 and a second amplifier A2 with variable gain a2.This gives a high-pass filtering means of the second order, because thelatter comprises a resonator RES with two variable parameters that arethe variable gains a1, a2.

The modulator MSD1 of FIG. 3 comprises two channels V1, V2 in order toloop back the digital output signal y(n) to respectively two negativeinputs of the summer SOM.

The first channel V1 comprises a first digital/analogue converter DAC1coupled between the output of the quantization means QTZ and the firstamplifier A1, the latter being coupled to the first negative input ofthe summer SOM.

The second channel V2 comprises a second digital/analogue converter DAC2coupled between a delay means RT and the second amplifier A2, the latterbeing coupled to the second negative input of the summer SOM. The delaymeans RT is also coupled between the output of the quantization meansQTZ and the second digital/analogue converter DAC2. This delay means RTmakes it possible to delay the digital signal y(n) leaving thequantization means QTZ by a period equal to half the sampling period Tsof the modulator MSD1.

It will be noted that a resonator is a system which oscillates naturallyat a precise frequency Fr which is its resonance frequency. The variablegains a1, a2 are determined as a function of the frequency Fe of theanalogue input signal x(t) in order to obtain a resonance frequency Frof the resonator that is approximately equal to half the samplingfrequency Fs of the modulator MSD1.

It will also be noted that, for a conventional high-pass filter, thegain is usually between 20 and 30. On the other hand, for a resonator,the gain is between 300 and 400. Moreover, the resonators are preciseand have a resonance difference of ±3 dB (decibels).

FIG. 4 illustrates a schematic representation of another embodiment ofthe continuous time analogue/digital converter. Certain elementsdescribed in the previous figures have also been transferred to thisFIG. 4. In this other embodiment, the filtering means FPH comprise twounits comprising respectively two resonators RES1, RES2 mounted inseries in order to obtain a fourth-order high-pass filtering means. Eachresonator is preceded by a summer SOM1, SOM2. The number of units, andin particular the number of resonators mounted in series, is notlimiting and it is possible to produce filtering means which comprisemore than two resonators in order to improve the precision of thefiltering means MF. However, if the number of resonators is increased,the processing time is also increased for the conversion of the analoguesignal x(t). This is why the use of two resonators provides a goodcompromise between precision and processing time for the digitalconversion.

Moreover, the filtering means MF comprise, in a first unit, a firstamplifier A1 with variable gain a1 and a second amplifier A2 withvariable gain a2. These filtering means MF also comprise, in a secondunit, a third amplifier A3 with variable gain a3 and a fourth amplifierA4 with variable gain a4.

As a non-limiting example, the continuous time analogue/digitalconverter may comprise a sigma delta modulator furnished with tworesonators to convert an analogue input signal having a frequency Fe=2GHz, with a sampling frequency Fs=4 GHz.

FIG. 5 shows schematically a wireless communication device APP. Thiswireless communication device APP comprises an antenna ANT fortransmitting and receiving signals for communication with a remote basestation. This device APP conventionally comprises a receive chain RXCHand a transmit chain, not shown here for simplification purposes.

The receive chain RXCH comprises an analogue portion comprising inparticular a low noise amplifier LNA connected to a continuous timeanalogue/digital converter CCT which comprises a sigma delta modulatorMSD1 as described above for converting the analogue input signal x(t)into a digital signal y(n) intended for a digital signal processor DSP.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A sigma delta modulator comprising: a firstsummer configured to add an analogue input signal to a feedback signalto generate a modified analogue signal; a high-pass filter configured tofilter the modified analogue signal to generate a filtered modifiedanalogue signal; and a quantizer configured to sample the filteredmodified analogue signal to generate a digital signal, the high-passfilter comprising an amplifier coupled between an output of thequantizer and an input of the first summer.
 2. The sigma delta modulatorof claim 1 further comprising a digital to analogue converter configuredto convert the digital signal to generate the feedback signal.
 3. Thesigma delta modulator of claim 1, wherein the high-pass filter has achopping frequency, and wherein the quantizer has sampling frequency,the chopping frequency being half of the sampling frequency.
 4. Thesigma delta modulator of claim 3, wherein the sampling frequency is 4GHz and the chopping frequency is 2 GHz.
 5. The sigma delta modulator ofclaim 1, wherein the high-pass filter further comprises a firstresonator.
 6. The sigma delta modulator of claim 5, wherein thehigh-pass filter further comprises a second resonator.
 7. The sigmadelta modulator of claim 1 further comprising a delay component coupledbetween an output of the quantizer and an input of the first summer. 8.A wireless communication device comprising: a sigma delta modulatorconfigured to receive an analogue input signal, the sigma deltamodulator comprising: a high-pass filter configured to filter theanalogue input signal to generate a filtered analogue signal; and aquantizer configured to sample the filtered analogue signal to generatea digital signal; a low noise amplifier comprising an input coupled toan antenna and an output coupled to the sigma delta modulator; and adigital signal processor configured to receive the digital signal fromthe sigma delta modulator.
 9. The wireless communication device of claim8, wherein the high-pass filter has a chopping frequency, and whereinthe quantizer has sampling frequency, the chopping frequency being lessthan the sampling frequency.
 10. The wireless communication device ofclaim 9, wherein the chopping frequency is equal to half of the samplingfrequency.
 11. The wireless communication device of claim 8 furthercomprising: at least one summer configured to add the analogue inputsignal to a feedback signal; and a digital to analogue converterconfigured to convert the digital signal to generate the feedbacksignal.
 12. The wireless communication device of claim 11 furthercomprising a delay component coupled between an output of the quantizerand an input of the at least one summer.
 13. The wireless communicationdevice of claim 8, wherein the high-pass filter comprises at least oneresonator and at least one variable-gain amplifier.
 14. The wirelesscommunication device of claim 8, wherein the quantizer has a samplingfrequency, the sampling frequency being equal to twice the frequency ofthe analogue input signal.
 15. A continuous time analogue/digitalconverter comprising: a sigma delta modulator configured to receive ananalogue input signal, the sigma delta modulator comprising: a quantizerhaving a sampling frequency; and a high-pass filter having a choppingfrequency, the chopping frequency being less than the samplingfrequency.
 16. The continuous time analogue/digital converter of claim15, wherein the sigma delta modulator further comprises at least onesummer, and wherein the high-pass filter comprises at least one unitincluding a resonator and a variable-gain amplifier coupled between anoutput of the quantizer and an input of the at least one summer, thevariable-gain amplifier having a gain set at a value wherein a resonancefrequency of the resonator is equal to the chopping frequency.
 17. Thecontinuous time analogue/digital converter of claim 16, wherein thehigh-pass filter comprises at least one other unit.
 18. The continuoustime analogue/digital converter of claim 16, wherein the sigma deltamodulator further comprises a delay component coupled between the outputof the quantizer and the input of the variable-gain amplifier, the delaycomponent being configured to delay a digital output signal of thequantizer by a period equal to half of the sampling period of thequantizer.
 19. The continuous time analogue/digital converter of claim15, wherein the chopping frequency is equal to half of the samplingfrequency.
 20. The continuous time analogue/digital converter of claim15, wherein the sigma delta modulator further comprises: at least onesummer having a first input coupled to the analogue input signal and anoutput coupled to the input of the high-pass filter, an output of thehigh-pass filter coupled to an input of the quantizer; and a digital toanalogue converter having an input coupled to an output of the quantizerand an output of the digital to analogue converter being coupled to asecond input of the at least one summer.